When sending data to a specific device on a network, the data must be forwarded by an intermediary device. Intermediary devices are often connected to multiple devices, and if each intermediary device forwards the data to all connected devices when sending data, the total amount of data sent increases, and the time it takes for each intermediary device to process the data also increases, leading to network delays.
The MAC address table installed in the L2 SWITCHES can solve this problem.
The MAC address table allows for the destination to be specified and FRAMES to be forwarded, enabling efficient data transfer. In the article "What is an In-Vehicle Ethernet SWITCHES?"/L2Routing, we provided an overview of the function of an L2 SWITCHES and the forwarding method used by an in-vehicle Ethernet switch.
This column provides a detailed explanation of the MAC address table and forwarding methods using MAC addresses.
MAC Address Table
What is a MAC Address?
A MAC address is a unique number written in hexadecimal that is assigned to network devices.
The upper 24 bits are the vendor number managed by IEEE, and the lower 24 bits are the product number managed by the vendor. Both are unique numbers, so each device's MAC address will not be duplicated.
In addition, it is assigned at the time of manufacturing and cannot be changed by the user.
What is a MAC Address Table?
The MAC address table is a database used for forwarding FRAMES. It associates the MAC addresses of devices connected to the L2 SWITCHES with the ports on the L2 SWITCHES.
There are two ways to register MAC addresses in the MAC address table: static MAC address learning, which is done manually, and dynamic MAC address learning, which is done automatically when FRAMES is received.
Registering and deleting MAC addresses
Static MAC Address Learning
You manually register MAC addresses in the MAC address table. You also manually delete MAC addresses. MAC addresses are not automatically deleted.
Dynamic MAC Address Learning
MAC addresses are automatically registered in the MAC address table. When a packet containing a source MAC address is sent to the L2 SWITCHES, the MAC address is added to the table only if it is not already registered in the MAC address table.
In addition, a function called aging automatically deletes MAC addresses over time.
Information about sending and receiving packets is provided in Ethernet FRAMES (Technical Column).
Packet transmission when multiple devices are connected
The behavior when sending a packet from Host_A to Host_D is as follows.
When registering a MAC address
If the destination MAC address is registered in the table, the L2 SWITCHES will only forward the packet to the corresponding port. This is called FILTERS.
When the MAC address is not registered
If the destination MAC address is not registered in the table, the L2 SWITCHES will forward the packet to all ports. This is called flooding. The source MAC address is registered in the address table. Also, if a host receives FRAMES is not addressed to it, it will discard the packet.
Packet transmission between L2 SWITCHES
Spanning Tree
To make a network redundant, multiple L2 SWITCHES are connected. If a MAC address is not registered in the address table, flooding may occur, which could cause a loop in part of the network. This is called a broadcast storm. As a result, the network bandwidth is compressed and network functionality goes down.
Spanning tree is a function that disables some ports to break loops in order to prevent broadcast storms. The protocol that performs this is called the Spanning Tree Protocol (STP) and is standardized in IEEE802.1D. Improved protocols of STP include RSTP and MSTP.
Link Aggregation
Connecting L2 SWITCHES with multiple CABLES will cause a loop, which will result in the line being blocked by STP. Ring aggregation is a function that aggregates lines by treating multiple ports as a single logical port, and is standardized in IEEE802.3ad.
The benefits of link aggregation are as follows:
- Increased bandwidth by aggregating lines
- Load balancing by using multiple lines
- Improved availability through line redundancy
Summary
This article explained the MAC address table and forwarding methods using MAC addresses. There are various forwarding methods depending on the state of the address table and the connection environment.
L2 SWITCHES also have functions to deal with situations where transfer is not possible.
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